It is common to mount multi-cylinder internal combustion engines within the hulls of boats, especially small boats, to provide propulsive power. Such engines can connect to propeller shafts mounted for rotation about axes fixed relative to the hull, or they can connect to steerable propeller arrangements or the like mounted to the exterior of the hull. Water-cooled automobile engines are commonly used in such boat contexts which are of two general kinds, namely, racing or high performance boats and general purpose or family boats. In boats of the first kind, the engine can be exposed, whereas in boats of the second kind, the engines are enclosed in housings which project into the passenger spaces of the boat. Where the engine is exposed, space and noise factors are not important, as a rule, and so an engine exhaust pipe can be provided for each engine cylinder and those pipes can project directly into the air, if desired. In general purpose or family boats, however, space, noise and temperature factors are quite important, and so engine exhaust systems in such boats are compact and water-cooled, and they are contained within the engine housing. The exhaust systems for engines in racing boats can be defined to maximize engine performance, but considerations of space and cost in general purpose boats severely limit the ability to affect engine performance through refinements in engine exhaust systems.
An exhaust system for an engine in a general purpose boat most commonly includes one or two exhaust manifolds connected directly to the engine, and a header pipe for each manifold. The header pipe connects to the manifold and extends either through the hull or to a further duct which extends through the hull. The manifolds and header pipes are water-jacketed, i.e., include in their walls passages through which water is pumped to cool the exhaust structures themselves as well as the engine exhaust gases which flow through them.
A need exists for exhaust systems, useful with engines mounted within the hulls of general purpose boats, which provide improvements in engine performance while also effectively observing the competing and limiting factors of space availability and cost.